1. Field of the Invention
The present invention relates to an image forming apparatus that includes a fixing device that fixes a formed image to a recording medium with heat, thereby performing a heat fixing operation.
2. Description of the Related Art
In recent years, there has been an increasing demand of the market to further shorten a first print time of image forming apparatuses, such as copiers, printers, facsimile machines, or multifunction peripherals. The first print time refers to the time that elapses from when a print request (image forming command) is received to when an image forming operation is performed after an operation to prepare for image forming is performed and then the first recording medium on which an image is formed is discharged. In image forming apparatuses that include a fixing device that performs a heat fixing operation to fix the formed image to a recording medium with heat, a warming-up time of the fixing device needs to be shortened in order to reduce the first print time. The warming-up time refers to the time taken to increase the temperature of a heating member to a target temperature (a target fixing temperature), whereby the heat fixing operation can be performed upon return from a fixing not-possible state (upon turn-on of the power source or upon return from a standby state).
Fixing devices that use a contact heating system, such as a heating roller system, film heating system, or electromagnetic induction heating system, are widely used to perform a heat fixing operation. Various conventional fixing devices with various systems have been proposed, where various measures are implemented to shorten the warming-up time. For example, in a fixing device with a film heating system, a thin fixing belt (a fixing film) that has a low heat capacity is heated, and the fixing film is brought into contact with a recording medium so that the image is fixed to the recording medium with heat. Thus, an advantage in shortening the warming-up time is produced (Japanese Patent Application Laid-open No. 2007-334205).
FIG. 12 is a schematic view that illustrates the relevant part of a conventional fixing device that uses a film heating system.
The fixing device includes an endless belt (a fixing belt or fixing film) 901, a metallic heat conductor 902 that has nearly a pipe-like shape and is installed within the endless belt 901, and a heat source 903 that is a heat source installed within the metallic heat conductor 902. The fixing device further includes a pressing roller 904 that is a pressing member that is in contact with the metallic heat conductor 902 via the endless belt 901 so as to form a nip section N. The opposing area of the metallic heat conductor 902 that is opposed to the pressing roller 904 is formed to be thinner than the other areas, and the outer circumference of the opposing area of the metallic heat conductor 902 is formed to have a flat surface. The endless belt 901 is rotated in accordance with the rotation of the pressing roller 904 and, at that time, the metallic heat conductor 902 guides the endless belt 901 to move. Furthermore, the endless belt 901 is heated via the metallic heat conductor 902 by the heat source 903 that is a heat source installed within the metallic heat conductor 902 so that the overall endless belt 901 can be heated. With the provision of the thin fixing belt that has a low heat capacity, the warming-up time can be shortened.
As described above, various measures have been implemented in the conventional fixing devices to shorten the warming-up time and, as a result, the first print time of the image forming apparatuses has been shortened. However, it is found out that, as the warming-up time of the fixing devices has been shortened, the following problem occurs.
The first print time of the image forming apparatus depends on the longest one of the start-up times of the operating units that are associated with an image forming operation. In conventional image forming apparatuses, the start-up time (warming-up time) of the fixing device is usually the longest one of the start-up times of the operating units that are associated with an image forming operation. Therefore, the first print time of the image forming apparatus has been reduced by shortening the start-up time of the fixing device.
However, in some cases, as the warming-up time of the fixing device has been shortened, the warming-up time of the fixing device is shorter than the start-up times of the other operating units, such as a controller (control unit). In such a case, the warming-up time of the fixing device elapses earlier than the start-up times of the other operating units. Therefore, it is necessary to keep the fixing temperature of the fixing device during a time period from when warming-up is completed to when the other operating units are started up so that a fixing operation is actually performed. As the fixing operation is not performed during the above time period, the electric power to keep the fixing temperature during the time period is wasted, which results in the problem of occurrence of wasted power consumption.
Especially, in electrophotographic color image forming apparatuses, for example, it is known that the target fixing temperature can be set to be low during a black-and-white mode in which images are formed by using black toner only compared to a color mode in which images are formed by using toner of multiple colors. This is because the amount of toner that adheres to a recording medium during the black-and-white mode is smaller than that during the color mode and therefore a lower fixing temperature is required to perform a fixing operation where toner is melted and softened. Therefore, during the black-and-white mode for which the target fixing temperature is lower, the warming-up time is shorter compared to the color mode for which the target fixing temperature is higher. Thus, the above-described problem is further noticeable during the black-and-white mode.
In consideration of the above-described problem, there is needed to provide an image forming apparatus that is capable of reducing a waste of power consumption that is required to keep the fixing temperature until the other operating units are started up.